1. Field of the Invention
This invention relates to a movable table unit which uses a piezoelectric element as a drive source to allow a movable table to move infinitesimally.
2. Description of the Related Art
Devices which move a table within an infinitesimal range through the use of the expanding/contracting motion of a piezoelectric element are conventionally known. Such devices comprise a movable table coupled to a stationary base through spring members, and cause a piezoelectric element to expand/contract for movement of the movable table relative to the stationary base. For example, in a device disclosed in Japanese Unexamined Patent Application Publication No. 2005-236166, a movable table can be moved by being mounted on a stationary base through spring members. The movable table is coupled to one end of a piezoelectric element and the stationary base is coupled to the other end of the piezoelectric element. Upon expansion/contraction of the piezoelectric element by the passage of an electric current through it, an expansion force or a contraction force resulting from the expansion or contraction acts on the movable table or the stationary base to move the movable table.
In such a device, a change in voltage applied to the piezoelectric element effects a change in the amount of expansion/contraction of the piezoelectric element, which in turn changes the amount of travel of the movable table. In consequence, the amount of travel of the movable table can be controlled by controlling the voltage applied to the piezoelectric element.
In actual fact, however, the relationship between the voltage applied to the piezoelectric element and the amount of expansion/contraction of the piezoelectric element exhibits hysteresis. Precise control of the amount of travel of the movable table cannot be easily achieved simply by controlling the voltage. To overcome this, a strain gauge is mounted on the piezoelectric element to detect the amount of strain of the piezoelectric element, and then the amount of travel of the movable table is calculated on the basis of the detection.
For obtaining, in this way, the amount of travel of the movable table from the detected value of the strain gauge mounted on the piezoelectric element, the relationship between the amount of strain of the piezoelectric element and the amount of travel of the movable table has previously been measured, and then based on the measured relationship the amount of displacement of the movable table is calculated.
For example, FIG. 9 shows a graph of the relationship between the amount of strain detected by the strain gauge mounted on the piezoelectric element and the amount of displacement of the movable table which is the amount of actual travel of the movable table.
The graph in FIG. 9 appears to be straight, but strictly speaking it is not straight, and is a graph showing varying displacements of the movable table between the directions in which the amount of strain of the piezoelectric element increases and decreases, that is, exhibiting the so-called hysteresis. From such a relationship as shown in FIG. 9, a straight line approximated to this graph is obtained by a least-squares method. FIG. 10 is a graph showing linear error in which the scales on the vertical axis represent the degree of error from the above-mentioned approximated straight line. It is seen from the linear error graph in FIG. 10 that there exists a linear error s1 which shows error from the approximated straight line and a back-and-forth difference s2 which is the difference between the amount of displacement of the movable table caused during the expansion of the piezoelectric element and the amount of displacement of the movable table caused during the contraction of the piezoelectric element.
In this regard, the conventional movable table unit has a strain gauge mounted on the piezoelectric element, and details of the structure of other components are omitted.
As described above, the relationship between the amount of strain of the piezoelectric element and the amount of displacement of the movable table exhibits hysteresis and is not a completely proportional relation. Such a relationship causes disadvantageous difficulties in precisely determining the amount of travel of the movable table.
In particular, some uses of the movable table unit using a piezoelectric element require a resolution of a few tens of nm. In such a case, the conventional movable table unit as described above is incapable of addressing the requirement.
To overcome this, combination with an optical detection means such as a linear scale or the like can be considered for detecting the amount of travel of the movable table and at the same time correcting the error. However, the provision of an optical detection means makes the apparatus complicated, with an increase in size and an increase in cost.